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Published OnlineFirst August 21, 2012; DOI:10.1158/1078-0432.CCR-12-2408
The Promise of Patient-Derived Xenografts: The Best Laid Plans of
Mice and Men
Scott Kopetz, Robert Lemos and Garth Powis
Clin Cancer Res 2012;18:5160-5162. Published OnlineFirst August 21, 2012.
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Copyright © 2012 American Association for Cancer Research
Published OnlineFirst August 21, 2012; DOI:10.1158/1078-0432.CCR-12-2408
Clinical
Cancer
Research
CCR Translations
Commentary on Julien et al., p. 5314
The Promise of Patient-Derived Xenografts: The Best Laid
Plans of Mice and Men
Scott Kopetz, Robert Lemos, and Garth Powis
Compared with xenografts from previously established cell lines, patient-derived xenografts may more
faithfully recapitulate the molecular diversity, cellular heterogeneity, and histology seen in patient tumors,
although other limitations of murine models remain. The ability of these models to inform clinical
development and answer mechanistic questions will determine their ultimate use. Clin Cancer Res; 18(19);
5160–2. Ó2012 AACR.
In this issue of Clinical Cancer Research, Julien and colleagues provide a detailed description of an extensive body
of work in patient-derived xenografts from patients with
colorectal cancer (1). Like many efforts described by Robert
Burns’ famous line in his poem "The Mouse," the ultimate
success of "The best laid plans of mice and men" remains to
be seen. In contrast to the most commonly used cell line–
xenograft model, patient-derived xenografts are established
from the immediate transfer of fresh tumor tissue from
patients into immunosuppressed mice. After a period of
stagnant or indolent growth, these xenografts enter a logarithmic growth phase suitable for harvesting and reimplantation in successive generations of mice with a high
tumor take rate. An enlarging body of work suggests patientderived xenografts may represent an informative model for
development of novel therapeutics, with the hope that they
can more faithfully predict the subsequent clinical success
and allow mechanistic studies of agent action that are not
possible in patients themselves. To fully use the potential of
this approach, however, fundamental model development
steps, such as those rigorously described by Julien and
colleagues, will be needed in a variety of tumors (1, 2).
The limitations of current preclinical models have been
well described and are increasingly cited as a key cause of the
low success rate of oncology drug development (3). Historically, panels of cell lines, many dating back decades,
have been used to screen novel therapeutics for clinical
activity and select tumor subtypes for further exploration.
In vivo studies will commonly use a small subset of these
cell line panels to identify tumor types of interest and to
make "go–no go" decisions for drug development. Previous
Authors' Affiliation: Department of Gastrointestinal Medical Oncology
and Experimental Therapeutics, The University of Texas MD Anderson
Cancer Center, Houston, Texas
Corresponding Author: Scott Kopetz, The University of Texas MD
Anderson Cancer Center, 1515 Holcombe Boulevard, Unit 426, Houston,
TX 77030. Phone: 713-792-2828; Fax: 713-563-6764; E-mail:
[email protected]
doi: 10.1158/1078-0432.CCR-12-2408
Ó2012 American Association for Cancer Research.
5160
coordinated efforts have been undertaken to improve the
predictive nature of these preclinical studies, as exemplified
by spheroid studies, which ultimately have not improved
preclinical predictions.
Increasingly over the past several years, patient-derived
xenografts, or tumorgrafts, have been used for proof-ofconcept studies (4). Ultimately, the success of this model
will come with time as it is integrated into development of
novel therapeutics. Several characteristics suggest these
models better recapitulate the human model. A review of
the features of the patient-derived xenografts that most
closely resemble this patient model is instructive to understand the strengths and weaknesses of this approach (Fig. 1).
The most commonly cited benefit of the patient-derived
xenograft model is the maintained intratumor heterogeneity and histologic characteristics seen in primary tumors.
Cell lines, and by extension cell line xenografts, undergo
extensive evolutionary selection through years of growth in
monolayers and rarely recapitulate the histology of parental
tumors when reimplanted. While patient-derived xenograft
studies, including the one presented in this issue, show
sustained histologic features over time, it is unclear whether
the molecular heterogeneity seen in early passages of the
tumor will be maintained over subsequent generations.
Earlier studies of in vitro colony formation from patient
tumors failed to provide clinical relevance, which was
attributed in part to the survival of only the most aggressive
tumor clones (5).
The second key and frequently cited feature is the ability
to maintain a component of human stroma in early
passages. Detailed evaluation of the stromal component,
however, suggests that there is less stroma in the earlypassage tumors than is seen in parental tumor, but that
this tumor-to-stromal ratio is sustained with subsequent
passages. Eventually, the human stroma is replaced by
stroma of murine origin, although the timing of this
remains to be further clarified. Although there is evidence
that the human-derived xenograft can be partially supported by murine stroma, differences in the ligand repertoire may be critical to the tumor phenotype. Gene expression profiles from early- and late-passage xenografts reflect
Clin Cancer Res; 18(19) October 1, 2012
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Copyright © 2012 American Association for Cancer Research
Published OnlineFirst August 21, 2012; DOI:10.1158/1078-0432.CCR-12-2408
Promise of Patient-Derived Xenografts
Cell lines
in vitro
Cell line xenografts
Patient-derived
xenografts
Patient with
refractory cancer
No
heterogeneity
Limited
intratumoral
heterogeneity
Modest diversity
of molecular
subtypes
Modest diversity
of molecular
subtypes
No stroma
Murine stroma;
No human stroma
Rapid growth
Rapid growth
Slower growth
Chronic growth
(doubling time
in days)
(doubling time
in days)
(doubling time
in weeks)
(doubling time in
months)
Untreated
Untreated
Mixed untreated
and previously
treated
Prior treatment
in all patients
No linked clinical
outcomes
No linked clinical
outcomes
Limited clinical
outcomes
available
Treatment
outcomes
available
Mixed primary and
metastatic sites
Mixed primary and
metastatic sites
Mixed primary
and metastatic
sites
No orthotopic
studies
Rarely orthotopic
studies
Rarely orthotopic
implantation
No immune
system
Limited immune
system
Higher intratumor
heterogeneity
Higher number
of molecular
subtypes
Admixed
murine/human
stroma
High intratumor
heterogeneity
Full range of
molecular
subtypes
Intact human
stroma
Metastatic sites
predominate
All orthotopic
(by definition)
Severely limited
immune system
Intact immune
system
© 2012 American Association for Cancer Research
Figure 1. Cell line–based models have been criticized for their poor ability to predict outcomes for advanced patients with cancer (3). Patient-derived
xenografts have several characteristics that better recapitulate the clinical reality for patients. This tumor model "abacus" highlights areas of strength and
weakness of patient-derived xenografts as compared with the cell line and human models. Blue squares further to the right represent positive characteristics
of the patient-derived xenografts that are closer to the tumor biology of the patient.
these changes in stromal characteristics but also provide
unique research opportunities to differentiate tumor and
stromal compartments through dedicated murine and
human arrays.
Cancer cell lines have been criticized for their modest
diversity of molecular subtypes and skewing toward subtypes of increased affinity to growth in a monolayer culture.
Profiling of tumors using the blunt metrics of mutational
frequency in this colorectal cancer xenograft set suggests
that a more complete spectrum of molecular subtypes may
be present. For example, tumors with a PI3KCA mutant and
KRAS/BRAF wild-type genotype were successfully established—although this subtype is exceedingly rare in the
www.aacrjournals.org
available colorectal cancer cell lines (6). However, for other
tumor types with lower overall engraftment rates, there may
be substantial biases induced in the biology of those tumors
that can be established. Further molecular profiling beyond
mutation spectrum will be required to characterize these
xenograft tumor banks.
Several current limitations of the patient-derived xenograft model as currently implemented can be addressed to
better recapitulate patients with refractory and metastatic
cancer. Many patient-derived xenografts are established
from the primary tumors of previously untreated patients
and thereby fail to reproduce a chemotherapy-refractory
patient population in whom most novel therapeutics will
Clin Cancer Res; 18(19) October 1, 2012
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Copyright © 2012 American Association for Cancer Research
5161
Published OnlineFirst August 21, 2012; DOI:10.1158/1078-0432.CCR-12-2408
Kopetz et al.
undergo their initial trials (7). Several series, including the
current bank of colorectal cancer xenografts described by
Julien and colleagues (1), are partially composed of tumors
from metastatic sites and reflect uncommon patients with
oligo-metastatic disease who underwent potentially curative surgical resection. Instead of relying on surgical specimens, biopsy samples can be engrafted, thereby extending
the application of the model to a wider range of patients and
improving the ability to study changes in the tumor at a time
when clinical resistance develops. Concerted efforts, including development of methods to reproducibly engraft biopsy
samples, will be needed to improve the clinical applicability
of the xenograft panels.
The critical limitation of either cell line or patient-derived
xenografts is the requirement to use immunosuppressed
mice. As an increasing number of clinical questions incorporate immune-based therapies, alternate models will need
to be pursued. In addition, several well-established limitations of murine models remain, including the imperfect
modeling of drug distribution and metabolism, and a more
rapid tumor growth rate than is seen in most patients with
cancer. Implantation of these tumors into the same organ
from which they were harvested may improve the clinical
relevance of these models but will require optimized methodology for labeling and imaging these tumors (8, 9).
The development of the patient-derived xenograft
models also introduces potential logistic challenges,
including assessment of the ability to freeze and reestablish tumors after months to years of storage, and the
minimization of warm ischemia time in more complex
surgical resections. Close coordination with the surgeons
and implantation of the specimens as rapidly as possible
after devascularization of the specimen improves engraftment rates and may be increasingly critical for tumors
with low engraftment rates.
Efforts to pair the clinical outcomes from patients and the
response of the corresponding xenografts have shown early
success (10, 11). Although the clinical use of prospective
efforts will be hampered by the time frame required for such
testing, the variable engraftment rates, and potential regulatory hurdles, it remains an important proof-of-principle
for the ability of xenografts to predict the efficacy of novel
therapeutics. Ultimately, validation of this approach will
come with successful selection of important biology amenable to pharmaceutical intervention and the corresponding ability to reduce early drug development failures. Only
then will we finally disprove the conclusion of Burns’ poem:
"The best laid plans of mice and men/Often go awry."
Disclosure of Potential Conflicts of Interest
No potential conflicts of interest were disclosed.
Authors' Contributions
Conception and design: S. Kopetz, R. Lemos, G. Powis
Development of methodology: R. Lemos, G. Powis, S. Kopetz
Acquisition of data (provided animals, acquired and managed patients,
provided facilities, etc.): G. Powis, S. Kopetz
Writing, review, and/or revision of the manuscript: S. Kopetz, G. Powis
Administrative, technical, or material support (i.e., reporting or organizing data, constructing databases): R. Lemos, S. Kopetz
Grant Support
This work was supported by CA136980 (S. Kopetz), CA95060 (S. Kopetz
and G. Powis), and a gift from the Perot Foundation (G. Powis).
Received August 9, 2012; accepted August 16, 2012; published OnlineFirst
August 21, 2012.
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